Aarhus University Seal / Aarhus Universitets segl

Ultracold Quantum Gases Group

Welcome to the ultracold quantum gas research group at Aarhus University!

In our research we investigate the properties of atomic gases at extremely low temperatures. This allows us to understand the fundamental quantum mechanical behaviour of few- and many-particle systems.


News


Atom number fluctuations in a BEC

Editors suggestion in Phys. Rev. Lett.: Observation of Atom Number Fluctuations in a Bose-Einstein Condensate

Fluctuations are a key property of both classical and quantum systems. While the fluctuations are well understood for many quantum systems at zero temperature, the case of an interacting quantum system at finite temperature still poses numerous challenges. Despite intense theoretical investigations of atom number fluctuations in Bose-Einstein condensates, their amplitude in experimentally relevant interacting systems is still not fully understood. Moreover, technical limitations have prevented their experimental investigation to date. Here we report the observation of these fluctuations. Our experiments are based on a stabilization technique, which allows for the preparation of ultracold thermal clouds at the shot noise level, thereby eliminating numerous technical noise sources. Furthermore, we make use of the correlations established by the evaporative cooling process to precisely determine the fluctuations and the sample temperature. This allows us to observe a telltale signature: the sudden increase in fluctuations of the condensate atom number close to the critical temperature.

Read our paper in Physical Review Letters or on arXiv.

(04/2019)


Schematic of the optical setup and the local probing scheme.

Spatially-selective magnetometry of ultracold atomic clouds

We demonstrate novel implementations of high-precision optical magnetometers which allow for spatially-selective and spatially-resolved in situ measurements using cold atomic clouds. These are realised by using shaped dispersive probe beams combined with spatially-resolved balanced homodyne detection. Two magnetometer sequences are discussed: a vectorial magnetometer, which yields sensitivities two orders of magnitude better compared to a previous realisation and a Larmor magnetometer capable of measuring absolute magnetic fields. We characterise the dependence of single-shot precision on the size of the analysed region for the vectorial magnetometer and provide a lower bound for the measurement precision of magnetic field gradients for the Larmor magnetometer. Finally, we give an outlook on how dynamic trapping potentials combined with selective probing can be used to realise enhanced quantum simulations in quantum gas microscopes.

Read our published paper in Journal of Physics B or on arXiv.

(03/2019)


Polaron energy across resonant interactions

Analyzing the Bose Polaron Across Resonant Interactions

Recently, two independent experiments reported the observation of long-lived polarons in a Bose-Einstein condensate, providing an excellent setting to study the generic scenario of a mobile impurity interacting with a quantum reservoir. Here, we expand the experimental analysis by disentangling the effects of trap inhomogeneities and the many-body continuum in one of these experiments. This makes it possible to extract the energy of the polaron at a well-defined density as a function of the interaction strength. Comparisons with quantum Monte-Carlo as well as diagrammatic calculations show good agreement, and provide a more detailed picture of the polaron properties at stronger interactions than previously possible. Moreover, we develop a semi-classical theory for the motional dynamics and three-body loss of the polarons, which partly explains a previously unresolved discrepancy between theory and experimental observations for repulsive interactions. Finally, we utilize quantum Monte-Carlo calculations to demonstrate that the findings reported in the two experiments are consistent with each other.

Read our paper on arXiv.

(12/2018)


Loss spectroscopy reveals the temperature dependence of an Efimov resonance.

Temperature dependence of an Efimov resonance in 39K

Ultracold atomic gases are an important testing ground for understanding few-body physics. In particular, these systems enable a detailed study of the Efimov effect. We use ultracold 39K to investigate the temperature dependence of an Efimov resonance. The shape and position of the observed resonance are analyzed by employing an empirical fit, and universal finite-temperature zero-range theory. Both procedures suggest that the resonance position shifts towards lower absolute scattering lengths when approaching the zero-temperature limit. We extrapolate this shift to obtain an estimate of the three-body parameter at zero temperature. A surprising finding of our study is that the resonance becomes less prominent at lower temperatures, which currently lacks a theoretical description and implies physical effects beyond available models. Finally, we present measurements performed near the Feshbach resonance center and discuss the prospects for observing the second Efimov resonance in 39K.

Read our published paper at PRA or on arXiv.

(11/2018)


Monopole oscillation frequency of a Lee-Huang-Yang fluid.

Dilute Fluid Governed by Quantum Fluctuations, published in PRL

Understanding the effects of interactions in complex quantum systems beyond the mean-field paradigm constitutes a fundamental problem in physics. Here, we show how the atom numbers and interactions in a Bose-Bose mixture can be tuned to cancel mean-field interactions completely. The resulting system is entirely governed by quantum fluctuations - specifically the Lee-Huang-Yang correlations. We derive an effective one-component Gross-Pitaevskii equation for this system, which is shown to be very accurate by comparison with a full two-component description. This allows us to show how the Lee-Huang-Yang correlation energy can be accurately measured using two powerful probes of atomic gases: collective excitations and radio-frequency spectroscopy. Importantly, the behavior of the system is robust against deviations from the atom number and interaction criteria for cancelling the mean-field interactions. This shows that it is feasible to realize a setting where quantum fluctuations are not masked by mean-field forces, allowing investigations of the Lee-Huang-Yang correction at unprecedented precision.

See also this news announcement.

Read our published paper in Physical Review Letters or on arXiv.

(10/2018)


Fabrice Gerbier

Fabrice Gerbier visits

Researcher Fabrice Gerbier from CNRS in France is visiting the department. He has recently studied the spacial coherence in a superfluid gas of bosonic atoms in an optical lattice. For independent atoms excited by a near-resonant laser, absorption-emission cycles destroy spatial coherences related to diffusion in momentum space. For strongly interacting bosons, Fabrice observed an anomalously slow coherence due to clustering of atoms.

Link to official lecture event.

(09/2018)


Gabriele Ferrari

Gabriele Ferrari visits

Researcher Gabriele Ferrari from Trento is visiting the department. By rapidly crossing the critical temperature to Bose-Einstein condensation, he has studied the growth of boundary defects, related to the Kibble-Zurek mechanism. These defects are identified as quantized vortices, and in his research, Gabriele has studied their real-time dynamics and interactions.

Link to official lecture event.

(09/2018)


Mick´s PhD defense - Atom Number Jumps in Ultracold Clouds

During his studies, Mick Althoff Kristensen has studied atom clouds cooled to ultra low temperatures. When a cloud of atoms is cooled to the lowest temperatures found anywhere in the universe, their quantum mechanical nature reveals itself. A prime example is the Bose-Einstein condensate, where the atoms accumulate in the quantum mechanical ground state and form a single large quantum object. Mick Althoff Kristensen has developed the most stable source of Bose-Einstein condensates, which has allowed him to study how atoms jump in and out of the condensate.

Official announcement.

(09/2018)


Funding

The Villum Foundation